1. Field of the Invention
The present invention relates to a nonreciprocal circuit device such as an isolator, a circulator, etc., which is employed for a high-frequency band, for example, a microwave band. More particularly, the invention relates to a nonreciprocal circuit device, which can achieve miniaturization and cost reduction when the device is used in mobile communication equipment.
2. Description of the Related Art
In general, a nonreciprocal circuit device such as a lumped-constant type isolator or circulator has characteristics in which the amount of attenuation of signals is extremely small in the direction along which signals are transmitted, whereas it is extremely large in the opposite direction. A conventional type of such an isolator includes a device having a structure shown in FIG. 7. The conventional isolator has a structure in which components including a permanent magnet 3; a spacer 14; a magnetic assembly 5 comprising three central conductors 51, 52, and 53, and a ferrite member 54; and a resin case 7 are arranged in a closed magnetic circuit formed primarily by an upper yoke 2 and a lower yoke 8. Ports P1 and P2 of the central conductors 51 and 52 are respectively connected to input/output terminals 71 and 72 and matching capacitors Co which are disposed in the resin case 7; a port P3 of the central conductor 53 is connected to another matching capacitor Co and a terminal resistor R; and an end of each matching capacitor Co and an end of the terminal resistor R are respectively connected to ground terminals 73.
The spacer 14 is disposed between the permanent magnet 3 and the magnetic assembly 5, and the upper yoke 2 is fitted into the lower yoke 8. In this state, the magnetic assembly 5 and a resin case 7 are pressed and secured into the lower yoke 8; the matching capacitors Co and the terminal resistor R are secured into the resin case 7; and the ports P1 to P3 of the respective central conductors 51 to 53 are secured to the matching capacitors Co, the terminal resistor R, and the input/output terminals 71 and 72, which are arranged in the resin case 7. In other words, the spacer 14 is used to fill in the space inside the nonreciprocal circuit device so as to achieve stable fixing of the components arranged in the nonreciprocal circuit device, for instance, the magnetic assembly 5, the matching capacitors Co, the terminal resistor R, etc.
FIG. 8 is an equivalent circuit diagram of the conventional isolator described above. As shown in this figure, the arrangement of the isolator is such that the matching capacitors Co are respectively connected to the ports P1 to P3, which are the ends of the central conductors 51 to 53, to form matching circuits and the port P3 is also connected to the terminal resistor R. Each inductor L has equivalent inductance, which is formed of the ferrite member 54 and the central conductors 51 to 53.
This type of isolator is employed in a shared-antenna transmitting/receiving circuit arranged in a mobile communication unit such as a mobile phone, a car cellular phone, etc. In this case, the isolator is surface-mounted on a mounting board comprising the circuit unit.
In general, an amplifier incorporated in such communication equipment has a non-linear characteristic, which causes extraneous radiation emissions (spurious radiation, particularly a second or third harmonic). The extraneous radiation emission is a factor which may trigger malfunctions in power amplifiers of other communication equipment or cause interference; the extraneous radiation emissions should therefore be reduced to be below a certain level.
In addition, since the isolator has a function of a band-pass filter in its transmission direction, attenuation of a signal in a frequency band outside its passband is large even in the transmission direction. However, an isolator is not originally intended for obtaining attenuation outside the passband; thus, the desired amount of attenuation in the frequency band where the extraneous radiation emissions occur (particularly, second and third harmonics) cannot be obtained by the above-described conventional isolator. Therefore, the conventional communication device uses an additional device for reducing extraneous radiation emissions, such as an extra filter, or the like.
Thus, when the conventional isolator is used, a filter for blocking extraneous radiation emissions (spurious emissions) is required. This causes problems in that use of the extra filter leads to increased cost due to the additional component and to increased size of the device. That is, the conventional device cannot meet demands for miniaturization and cost reduction.